Abstract
This paper examines the structure of /iu/ and /ui/ in Sixian Hakka by means of phonological processes, syllable contraction, co-occurrence restrictions, rhyming patterns, and phonetic measurement. Except co-occurrence restrictions, which fail to reflect the compositional structure of the two diphthongs under investigation, all other linguistic evidence shows that /iu/ is a falling diphthong, and /ui/ is a rising diphthong. The result has two theoretical implications. One indicates that the X-theory (Levin 1985) together with a language-specific sonority hierarchy pinpoints the syllable structure of Sixian Hakka. The other reveals that Kiparsky's (1979) sonority scale of a > ε > o > i > u holds true in Sixian Hakka.